There is substantial interest in using ceramics and ceramic composites in a variety of industrial, electrical, and structural applications. Numerous properties characteristic of these materials, such as hardness, refractoriness, thermal and electrical insulation, and resistance to erosion and corrosion, may be advantageously and beneficially utilized depending on the end-use. Also, ceramics and ceramic composites provide attractive alternatives to metals for manye xisting purposes, as well as enabling the development of new components for which metals or other materials are unsuitable.
There are several limitations, however, in substituting ceramics for metals, and the development and production of ceramic components for technologically advanced applications is attended with problems. Known methods of preparing ceramic components involves powder-based fabrication, most typically at elevated tempertures and pressures, such as by hot-pressing, reaction sintering and reaction hot-pressing. This technology for fabricating ceramics manifests numerous deficiencies. These limitations or deficiencies include, for example, scaling versatility, capability to produce complex shapes, high costs of sinterable powders, lack of batch-to-batch reproducibility of powder properties, and substantial shrinkage on sintering. The present invention overcomes these limitations or deficiencies, and provides a novel method for reliably producing refractory metal carbide composites.
Ceramic carbides are well known in the art, and have been extensively studied in the ceramics industry. Also, components of these materials, made by conventional powder processing techniques, have achieved limited commercial success. A different process has been developed for the manufacture of siliconized silicon carbide, which produces a self-bonded ceramic body. In one such process known as the REFEL process, molten silicon is caused to infiltrate a porous preform of carbon and silicon carbide. The molten silicon reacts with the carbon to form additional silicon carbide that partially fills the interstices of the preform. The resulting ceramic components are relatively dense and brittle, consisting of silicon carbide and silicon. Although this process has become well known and there is extensive patent coverage, there is no suggestion that the REFEL process or other related processes are applicable to other elements or metals. In fact, silicon is the only element of Group IVA of the Periodic Table (C,Si,Ge,Sn,Pb) that forms a ceramic carbide by reaction of the molten lement with carbon, and therefore there is no reason to believe that other metals can be used in a similar process. (Any reference to the Periodic Table is from the " Handbook of Chemistry and Physics", 59th Edition, 1978-1979, CRC Press, Inc.,)
High temperature resistant articles are disclosed in U.S. Pat. No. 3,288,573 to Abos. In accordance with the teach ings of this patent, there is disclosed a composite comprised of graphite particles surrounded by an envelope of a carbide-forming material, including titanium, zirconium, hafnium, vanadium, nickel, tantalum, chromium, molybdenum, tungsten and silicon. According to the process of this patent, a preheated porous graphite body is infiltrated by a molten mass of silicon, or other identified metal, which partially reacts with the graphite particles to form carbide envelopes around each particle. Because the resulting product conains free carbon, the product exhibits certain qualities of graphite, most notably thermal shock resistance.
Among materials having potentially superior properties for particular components are the carbides of the Group IVB metals, viz., titanium, zirconium, and hafnium. It is known to produce titanium, zirconium and hafnium carbides by a method known as self-propagating high temperature synthesis, in which a powder mixture of the metal and carbon is ignited by local heating so that the resulting combustion front sweeps through the mixture resulting in the formation of the metal carbide. A major disadvantage of this method, however, is that upon combustion of adsorbed contaminants there is a virorous evolution of gases which causes a porous and inhomogeneous microstructure. Porosity also may be caused by melting of the reaction product in the intense heat of the reaction, followed by local shrinkage on solidification. In some instances, an improvement in microstructure can be achieved by application of pressure during combustion.